Chemical hydrides for hydrogen storage are being explored as alternatives to high-pressure hydrogen tanks, sorbents, adsorbed hydrogen, and metal hydride fuels. Chemical hydrides could be packaged as non-pyrophoric, non-hazardous, solid, slurried or liquid fuels for automotive applications. Hydrogen may then be generated from such chemical hydrides under controlled conditions on-board and on demand. The spent fuel may then be regenerated either on-board or off-board.
Hydrogen storage materials ideally have a high hydrogen content and low molecular weight. Ammonia-borane (“H3NBH3”), which has a hydrogen storage capacity of 19.6 wt %, is an attractive material for such applications. Because the ammonia borane molecule contains both hydridic and protic hydrogen atoms, it spontaneously loses H2 at temperatures above 80° C. H3NBH3 can be dehydrogenated completely, forming ceramic BN, but temperatures in excess of 500° C. are typically needed. Thermal decomposition of ammonia-borane in solution initially affords the cyclic oligomers cyclotriborazane (“B3N3H12”) and borazine (“B3N3H6”). B3N3H6 may be prepared from ammonia borane on a large scale in high yield over 3 hours by simply heating a tetraglyme solution of ammonia borane. It also has been shown that borazine can be thermally crosslinked at temperatures as low as 70° C. with concomitant H2 evolution.
It is possible to obtain a large amount of hydrogen from H3NBH3, but low energy (i.e., minimal heat input) methods to utilize this fuel are only just being developed. For example, H2 has been liberated at room temperature from H3NBH3 and the related species dimethylamine-borane (HMe2NBH3) by adding precious metal catalysts. For example, select Rh(I) species dehydrocouple HMe2NBH3 to form H2, along with the cyclic dimer [Me2NBH2]2 and acyclic aminoborane polymers. Phosphine-boranes having the general formula H2RPBH3 (R═H, Ph) also can be dehydrocoupled using metal catalysts to yield acyclic polymers.
Current methods for dehydrogenating amine-boranes at low temperatures require expensive precious metal catalysts such as rhodium. Hydrogen production using amine borane without the use of precious metal catalysts at low temperatures is desirable, as are less expensive, base metal-containing catalysts useful for producing hydrogen from ammonia borane.